1. Field of the Invention
The present invention relates to a flow rate sensor for detecting the flow rate (amount or velocity) of a fluid and, more particularly, to a thermosensitive flow rate sensor suited for detecting the amount of air flowing into an internal-combustion engine employed for an automobile or the like.
2. Description of the Related Art
Generally, in an electronically controlled fuel injection system in an automotive engine, it is necessary to accurately measure the amount of air introduced into an engine to control the air-fuel ratio of an air fuel mixture. For this reason, a thermosensitive flow rate sensor that provides a mass flow signal and minimizes a pressure loss at the same time has been frequently used as an air flow rate detector in recent years.
FIG. 24 is a front view showing a thermosensitive flow rate sensor, which has been disclosed in Japanese Unexamined Patent Publication (Laid-open) No. 6-265384, observed from the upstream side, and FIG. 25 is a sectional side view thereof. FIG. 26 shows the piping of an induction or intake system in an automotive engine to which the thermosensitive flow rate sensor has been attached.
Referring to FIG. 26, reference numeral 1 denotes a main pipe through which the air having passed through an air cleaner 15 incorporating an air cleaner element 16 flows, the main pipe 1 having a downstream end thereof connected to an engine (not shown) via an air intake duct 17. Installed inside the main pipe 1 is a flow rate detector 5 equipped with a flow rate detection element 4 composed of a temperature compensation resistor 9 for measuring the temperature of intake air and a heating resistor 10 which is adapted to be heated to a predetermined temperature. Thermosensitive resistors made of platinum or the like are used for the temperature compensation resistor 9 and the heating resistor 10. The flow rate detection element 4 is electrically connected to a control circuit 7 via a terminal 6. As shown in FIG. 26, the air flow that has passed through the air cleaner 15 flows into the main pipe 1. At that time, the amount of heat determined on the basis of the amount of the intake air is taken away from the heating resistor 10. This should naturally lower the temperature of the heating resistor 10, but the control circuit 7 controls the heating current supplied to the heating resistor 10 in order to maintain the temperature difference from the temperature of the intake air measured by the temperature compensation resistor 9 at a nearly constant level. This makes it possible to measure the amount of intake air from the value of the heating current supplied to the heating resistor 10.
In an automotive four-stroke cycle engine, however, the positive pressure occurring at the exhaust side at the time of a so-called "overlap" where both an intake valve and an exhaust valve are open causes the occurrence of a pulsed flow including a reverse flow to the intake side, depending on the construction of the induction pipe composed primarily of the main pipe 1 and the air intake duct 17, and/or depending on the degree of opening of a throttle valve that is adapted to be opened and closed according to the depression and release of an accelerator pedal. There has been a problem in that the foregoing flow rate detection element 4 does not have a function of detecting the direction of an air flow, leading to an increased flow rate detection error.
To solve such a problem, efforts have been made to develop a flow rate detection element capable of detecting the direction of flow of a fluid, as disclosed in Japanese Unexamined Patent Publication No. 1-185416. FIG. 27 shows the composition of the flow rate detection element 4, and FIG. 28 is a perspective view of a flow rate detecting section 5 in which the flow rate detection element 4 has been installed. FIG. 29 shows the configuration of a control circuit for detecting the flow rate and the direction of flow.
The flow rate detection element 4 uses a pair of heating resistors 10a, 10b, and a pair of temperature compensation resistors 9a, 9b. The heating resistors 10a, 10b and the temperature compensation resistors 9a, 9b composed of platinum thin films are formed on a plate-shaped substrate made of an electrically insulated material such as a ceramic, which has high heat conductivity, by means of sputtering and photoetching. The parts located on the intake upstream side will be denoted by a subscript "a" while the parts located on the intake downstream side will be denoted by a subscript "b". As shown in FIG. 29, the respective heating resistors 10a and 10b are set so that their temperature is set to a value higher than the temperature of the intake air by a predetermined value. The heating current is controlled by a differential amplifier 12 and a transistor 13 so that the resistance value Rh of the heating resistors 10a and 10b remains at a constant value regardless of the flow rate, thus making it possible to provide a voltage value matched to the amount of intake air under the action of reference resistors 11a and 11b. In general, the thermal equilibrium between the heating resistors 10a, 10b and a fluid is given by the following formula: EQU Q=h.multidot.S.multidot..DELTA.T
where Q: Amount of heat radiated from heating resistors PA1 a support member having first and second guide surfaces formed on opposite sides thereof for guiding the fluid therealong; and PA1 a flow rate detection element provided on the second guide surface of the support member; PA1 wherein the second guide surface of the support member on which the flow rate detection element is provided is formed by extending it longer than the first guide surface in a downstream direction. PA1 a support member having a surface for guiding the fluid therealong, PA1 a flow rate detection element provided on the guide surface of the support member for detecting the flow rate of the fluid; and PA1 a restraining portion provided on a downstream side of the support member for restraining occurrence of a vortex.
h: Heat conductivity PA2 S: Surface area of heating resistors PA2 .DELTA.T: Temperature difference between heating resistors and fluid
In the case of a flow in the forward direction, if the flow rate is constant, then the heat conductivity h1 of the heating resistor 10a located on the upstream side becomes larger than the heat conductivity h2 of the heating resistor 10b located on the downstream side. Hence, the output Va of the reference resistor 11a becomes larger than the output Vb of the reference resistor 11b. In the case of a flow in the reverse direction, the aspect of the heat transfer is reversed from that of the forward direction; hence, h1&lt;h2 results, and the outputs of the reference resistors become Va&lt;Vb. Accordingly, both the output voltages Va and Vb are turned into a differential output by using a comparator 14 thereby to enable the detection of the flow rate and the direction of flow.
This type of thermosensitive flow rate sensor, however, is not yet satisfactory for detecting the flow rate of a pulsating intake flow. For instance, especially at the time of deceleration of a large pulsating flow not accompanied by a reverse flow, a local reverse vortex is generated around the flow rate detection element 4, and the flow rate detection element 4 undesirably detects this local reverse flow. This has been posing a problem of deteriorated accuracy in detecting the amount of air flow passing through the main pipe 1.